Impact of community asymptomatic rapid antigen testing on covid-19 related hospital admissions: synthetic control study

Abstract Objective To analyse the impact of voluntary rapid testing for SARS-CoV-2 antigen in Liverpool city on covid-19 related hospital admissions. Design Synthetic control analysis comparing hospital admissions for small areas in the intervention population with a group of control areas weighted to be similar for past covid-19 related hospital admission rates and sociodemographic factors. Setting Liverpool city, UK, 6 November 2020 to 2 January 2021, under the intervention of Covid-SMART (systematic meaningful asymptomatic repeated testing) voluntary, open access supervised self-testing with lateral flow devices, compared with control areas selected from the rest of England. Population General population of Liverpool (n=498 042) and a synthetic control population from the rest of England. Main outcome measure Weekly covid-19 related hospital admissions for neighbourhoods in England. Results The introduction of community testing was associated with a 43% (95% confidence interval 29% to 57%) reduction (146 (96 to 192) in total) in covid-19 related hospital admissions in Liverpool compared with the synthetic control population (non-adjacent set of neighbourhoods with aggregate trends in covid-19 hospital admissions similar to Liverpool) for the initial period of intensive testing with military assistance in national lockdown from 6 November to 3 December 2020. A 25% (11% to 35%) reduction (239 (104 to 333) in total) was estimated across the overall intervention period (6 November 2020 to 2 January 2021), involving fewer testing centres, before England’s national roll-out of community testing, after adjusting for regional differences in tiers of covid-19 restrictions from 3 December 2020 to 2 January 2021. Conclusions The city-wide pilot of community based asymptomatic testing for SARS-CoV-2 was associated with substantially reduced covid-19 related hospital admissions. Large scale asymptomatic rapid testing for SARS-CoV-2 could help reduce transmission and prevent hospital admissions.


Part 2: Estimating Tier 3 restrictions on SARS-CoV-2 transmission compared to entering Tier 2 restrictions on 3 December 2020.
We analysed the impact of Tier 3 restrictions on SARS-CoV-2 transmission introduced in December 2020 in England, compared to Tier 2 restrictions. The main differences of Tier 2 and 3 were additional restrictions on meeting people outdoors and restrictions on the hospitality sector. To be more specific, in Tier 3 people were prohibited from meeting with people outside their household in private gardens whilst in Tier 2 people were allowed to meet with up to six people in private gardens; pubs and restaurants were closed in Tier 3 areas whilst in Tier 2 areas only those serving food remained open. We used data on COVID-19 restrictions that were compiled and made available by the Open Data Institute. [1] As Liverpool introduced the less restrictive Tier 2 measures on 3 rd December 2020, whilst new tier's restrictions typically took effect in many local authorities on the first forthcoming Monday (7 th December 2020), we therefore specified 2 weeks after that point as the earliest plausible time at which the change in restrictions could affect hospital admissions (20 th December 2020). We investigated the change in MSOAlevel COVID-19 hospital admissions in the intervention group (Tier 3 areas) using synthetic control analysis, 12 weeks before and 10 weeks after that time point, compared to a synthetic control group derived from places that entered Tier 2 at the same time. The exact time frame ranged from 4 October 2020 to 21 February 2021.
We identified 2,809 Tier 3 MSOAs as the intervention group, whilst the synthetic control group was constructed from the 3,481 Tier 2 MSOAs (excluding 61 MSOAs in the Local Authority of Liverpool).
Along with the local area characteristics outlined above in our analysis of COVID-SMART, we additionally accounted for differences in the prevalence of the new variant B.1.1.7 (Alpha), which became dominant during that time period, by including the proportion of positive tests with S-gene failure on PCR testing for each local authority in the period before the 20 th December using data from Public Health England.
[23] To further control for the confounding effect of community testing, we also excluded the 142 MSOAs (2.2% of all non-intervention MSOAs) from the control group if they were within authorities with a mean LFT testing rate of more than 1 per 100 population per week between 6 th November and 2 nd January 2021. In the sensitivity testing in Part 5 of the Supplement, we provided the estimation of the Tier 3 effect upon hospitalisation, without excluding these 142 MSOAs.
The introduction of Tier 3 restrictions in December was associated with an average reduction of hospital admissions of 17% (95% CI 13% to 21%) in Tier 3 areas compared to Tier 2 areas. In the sensitivity testing in Part 5 of the Supplement, without excluding the 142 MSOAs with a mean LFT testing rate of more than 1 per 100 population per week during the pilot period of the community testing in Liverpool, the introduction of Tier 3 restrictions in December was associated with an average reduction of hospital admissions of 14% (95% CI 9% to 17%) in Tier 3 areas compared to Tier 2 areas.
As can be seen from Figure SF3 the effect of the Tier restrictions changes over time.
In adjusting the MSOA hospital admissions in Tier 3 areas to account for this in our analysis of COVID-SMART, we applied the weekly effect sizes from the synthetic control analysis of Tiered restrictions.    Table 2

Part 7: Sensitivity test of local authority level synthetic control analysis.
Our approach using the synthetic control method for micro data developed by Robbins et al, [3] aggregates a large number of small areas to provide a synthetic control group similar to Liverpool. As we note our synthetic control group was made up of a nonadjacent set of neighbourhoods, that in aggregate had followed similar trends in hospitalisation to the contiguous neighbourhoods of Liverpool. However, this could introduce bias as a combination of those smaller areas may not mimic rates in a contiguous urban area. We therefore conducted a sensitivity analysis at the local authority level, matching Liverpool to 10 other local authority areas that have similar characteristics and previous trends in hospital admissions.
As for this analysis we have just one intervention unit, this presents estimation complications in deriving a relevant comparison group and for calculating standard errors. We therefore use the synthetic control approach developed by Brodersen et al, [4] which has previously been used to evaluate local authority level policies with single intervention units. The control local authorities for this analysis were selected using Mahalanobis nearest neighbour matching, [6,7] matching on the same variables included in our main analysis aggregated to the local authority level and applying the same exclusion criteria. This provided 10 local authorities that were included in the comparison group: Stockton-on-Tees, County Durham, Burnley, Preston, West Lancashire, Wyre, Bury, Stockport, Liverpool, Newcastle upon Tyne and Sunderland. We used the same intervention period from 6 th November to 3 rd December 2020 in this analysis as in our main analysis.
The results in Table SF5 indicate an effect size that is consistent with our main analysis using small area data, with the intervention during the initial period being associated with a 68% reduction in hospital admissions (95% credible intervals 116% to 18% reduction, p=0.009). Figure SF5 shows the trend in Liverpool and the estimated counterfactual trend and Figure SF6 shows the cumulative difference between the two.   Part 9: Sensitivity test of using the score of Education Skills and Training Deprivation rather than the composite score of IMD in the matching.